1. Field of the Invention
The present invention relates to an image forming apparatus using a liquid developer including a carrier liquid with toner particles dispersed in solvent.
2. Description of the Related Art
An electrophotographic recording apparatus using a liquid developer has been revalued in recent years. An electrophotographic recording apparatus has advantages not realized by a dry-type electrophotographic recording apparatus, for example, high picture quality equal to offset printing by using very fine submicron toner particles, low copy cost with sufficient image density due to small amount of toner particles, and energy-saving by fixing toner particles to a recording paper sheet at a relatively low temperature.
As a transfer system of an electrophotographic recording apparatus using a liquid developer, a type using a shearing force is proposed by Jpn. Pat. Appln. KOKAI Publication No. 2000-347520. This system increases the transferability by giving a shearing force to a visible image.
It is necessary for giving a visible image a sufficient shearing force to increase a condensing force of toner particles by eliminating a surplus carrier liquid remained in and close to a visible image formed on a photosensitive member.
As a known method of eliminating the surplus carrier liquid, there is a roller absorbing method which absorbs a carrier liquid by forming a nip by making contact between a porous member formed on the outside of a roller contact and a photosensitive member.
The roller absorbing method has an advantage of increasing and maintaining the absorbing force of a porous member by providing a means for pulling in a surplus carrier liquid absorbed by the porous member. This method can be enhanced in the speed compared with the other methods of eliminating a surplus carrier liquid. Further, the method does not stain the inside of an apparatus, and saves energy. A surplus carrier liquid can be collected as a liquid, which makes it very suitable for recycling the solvent. Clogging of the porous member can be prevented by providing a cleaning roller for cleaning the porous member, and the carrier liquid elimination performance can be held stably for a long period.
However, the roller absorbing method has such problems as a speed difference (relative speed) between the porous member and photosensitive member, and a transfer of a visible image on the photosensitive member to the roller by an excessive pressing force, causing a disturbance in a visible image. A disturbance in a visible image (hereinafter called a ghost image) is also caused due to the visible image adhering to the absorbing roller being transferred again to the photosensitive member.
Many proposals have been made for the roller absorbing method. In particular, Jpn. Pat. Appln. KOKAI Publication No.7-225516 proposed to contact a porous absorbing roller covered with a porous layer with electrical conductivity and elasticity to a breathable porous sleeve with electric conductivity and rigidity, so as to form an even nip in substantially the total width of a photosensitive member, and rotate the roller as a follower of the sleeve.
This method has an advantage that a speed difference (relative speed) is not generated between the porous absorbing roller and photosensitive member. Further, to keep the nip constant, a tracking roller is provided at both ends of the porous absorbing roller, to contact the photosensitive member and keep the distance constant.
However, even in the method disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 7-225516, a pressing force in the nip is increased to obtain a nip capable of transmitting a driving force to the porous absorbing roller without generating a relative speed (speed difference) between the porous member and photosensitive member. Further, the nip is evenly formed in substantially the total width of the photosensitive member including the part to form a visible image, giving a large pressing force also to a visible image. This raises a problem that a ghost image (a visible image adhered to the porous absorbing roller is transferred again to the photosensitive member) is not prevented.